It is well known to use ultrasonic energy to prepare sample materials for diagnostic investigation. When analyzing a sample material, particularly a sample consisting of two or more materials, it is important that the sample be as homogeneously mixed as possible. Such thorough and complete mixing tends to provide more consistent and accurate diagnostics of the sample.
Ultrasonic energy is used to provide complete and homogeneous mixing of liquid samples. This is generally accomplished by inserting a probe, generally made of a metal such as titanium, into a sample vessel containing the material sample and then vibrating the probe at an ultrasonic frequency. Such vibration, typically between 20 kHz and 40 kHz, causes “cavitation” to occur in the liquid sample. Cavitation refers to the rapid formation and collapse of vapor pockets in a liquid in regions of very low pressure. In the case of ultrasonic mixing, the regions of very low pressure are formed by the rapid oscillation of the probe. The vapor pockets, or bubbles, quickly expand and contract, thereby providing effective mixing of the components of the sample. Once the ultrasonic mixing of the sample is complete, the probe is removed from the sample, cleaned, and may then be used to mix another sample.
This process has significant drawbacks, however. First, the probe will often erode due to repeated contact with the material samples to an extent such that particles of the metal out of which the probe is formed, typically titanium, will contaminate the sample. Such contamination is often significant enough to affect the results of subsequent analysis of the sample. It is of great importance to maintain the purity of the samples so that the results of the analysis will be accurate.
Second, probes which do erode as a result of direct contact with many liquid samples over time need to be replaced regularly. Ultrasonic probes are precision parts which are not inexpensive. Therefore, frequent replacement of the probes is to be avoided if possible.
Third, the ultrasonic probes must be thoroughly cleaned between each sample mixing so as to avoid contamination of a subsequent sample by material from an earlier sample. It significantly increases the amount of time required to process multiple samples in series when the probe must be cleaned before each use. Thus, in automated systems for mixing many samples in series, the process can be extremely time-consuming.
What is needed then, is a system and method for utilizing ultrasonic mixing which minimizes the above-described drawbacks of traditional ultrasonic mixing.